A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin (Sn)), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
In an alternative arrangement a radiation system for producing EUV radiation may use an electrical discharge to generate the plasma. The electrical discharge passes into a gas or vapor such as Xe gas, Li vapor or Sn vapor, generating a very hot plasma which emits EUV radiation. Such a radiation system is typically termed a discharge produced plasma (DPP) source.
Plasma creation within an EUV source may cause contamination particles to be created from the fuel. These contamination particles may move either at relatively fast speeds, in which case they tend to generally follow the path of the radiation beam; or at relatively slow speeds, in which case they are free to undergo Brownian motion. In some lithographic apparatus the relatively slow moving contamination particles may be conveyed by a flow of gas within the lithographic apparatus.
Both the relatively fast moving and relatively slow moving contamination particles may move towards the patterning device of the lithographic apparatus. If the contamination particles reach the patterning device (even in very small numbers) then they may contaminate the patterning device. The contamination of the patterning device may reduce the imaging performance of the lithographic apparatus and may in more serious cases require the patterning device to be replaced. The patterning device can be expensive and therefore any reduction in the frequency with which it must be replaced may be advantageous. Furthermore, replacement of the patterning device is a time consuming process, during which the operation of the lithographic apparatus may have to be stopped. Stopping the operation of the lithographic apparatus may reduce the output of the lithographic apparatus and thereby reduce its efficiency, which is undesirable.
It is desirable to provide a lithographic apparatus which can capture both fast and slow moving contamination particles such that they are less likely to contaminate the patterning device.
According to an aspect of the invention, there is provided a lithographic apparatus that includes a radiation source configured to produce a radiation beam, and a support configured to support a patterning device. The patterning device is configured to impart the radiation beam with a pattern to form a patterned radiation beam. A chamber is located between the radiation source and the support. The chamber contains at least one optical component configured to reflect the radiation beam. The chamber is configured to permit radiation from the radiation source to pass therethrough. A membrane defines part of the chamber. The membrane is configured to permit the passage of the radiation beam through the membrane, and to prevent the passage of contamination particles through the membrane. A particle trapping structure is configured to permit gas to flow along an indirect path from inside the chamber to outside the chamber, the indirect path of the particle trapping structure being configured to substantially prevent the passage of contamination particles from inside the chamber to outside the chamber.
According to an aspect of the invention, there is provided a lithographic method that includes generating a radiation beam, and directing the radiation beam through a chamber containing at least one optical component that reflects the radiation beam. The radiation beam is directed towards a patterning device. The chamber includes a membrane. The method includes preventing the passage of contamination particles with the membrane when the radiation beam passes from the chamber through the membrane and towards the patterning device, flowing gas from inside the chamber to outside the chamber along an indirect path through a particle trapping structure, the indirect path substantially preventing the passage of contamination particles from inside the chamber to outside the chamber, imparting the radiation beam with a pattern to form a patterned radiation beam with the patterning device, and projecting the patterned beam of radiation onto a substrate with a projection system.
According to an aspect of the invention there is provided a lithographic apparatus comprising a radiation source configured to produce a radiation beam and a support configured to support a patterning device, the patterning device being configured to impart the radiation beam with a pattern to form a patterned radiation beam, wherein the support is provided with a pellicle which comprises a layer of graphene.
According to an aspect of the invention there is provided a spectral purity filter comprising a grid configured to prevent or reduce the passage of infrared radiation, wherein the grid is covered with graphene which prevents the passage of oxygen to the grid. The graphene may be provided as one or more layers, or may surround ribs of the grid.
According to an aspect of the invention there is provided a spectral purity filter comprising a grid configured to prevent or reduce the passage of infrared radiation, the grid comprising a tungsten/graphene multi-layered structure.
According to an aspect of the invention there is provided a spectral purity filter comprising a material which blocks out-of-band radiation, wherein the spectral purity filter further comprises a layer of graphene which supports the material.
According to an aspect of the invention there is provided a multi-layer mirror comprising alternating layers of a first material and a second material, wherein graphene is provided between the alternating layers.
According to an aspect of the invention there is provided a multi-layer mirror comprising alternating layers of a first material and a second material, wherein a layer of graphene is provided as an outer layer of the multi-layer mirror.
According to an aspect of the invention there is provided a lithographic apparatus having a graphene membrane which is configured to stop the passage of contamination particles and to transmit EUV radiation.